The term Iaidō (居合道) refers to a specific style of kenjutsu that is concerned with the smooth, controlled movements of drawing the sword from its scabbard, striking or cutting an opponent, removing blood from the blade, and then replacing the sword in the scabbard. The linear motion and force applied to draw the sword from the scabbard results in slashes that are faster and stronger than ones executed with the sword already drawn.
The speed at which these techniques can be executed — the initial strike in particular — is such, that when performed by a skilled user, it can prevent enemy shinobi from forming hand seals (and thus ninjutsu), by exploiting the small opening that these movements create. This makes them highly effective against most shinobi and potentially, those of even Hanzō's calibre, if performed by someone such as Mifune who has mastered the style.
Izuna Uchiha's sword skills were shown similar to this style.